how to overlay an element in css code example

Example 1: css div overlay div

#container { //goes to the bottom
  width: 100px;
  height: 100px;
  position: relative;
}
#div1{ //comes to the top
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  z-index: 10;
}

Example 2: overlay a box in css

html, body{
  min-height: 100%;
}
body{
  position: relative;
}
.overlay{
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 10;
  background-color: rgba(0,0,0,0.5); /*dim the background*/
}